An exhaust aftertreatment device that purifies nitrogen oxides (NOx) contained in exhaust gas of an engine with a selective catalytic reduction (abbreviated as “SCR” hereinafter) has been known. Urea aqueous solution injected by an injector is supplied to the SCR. The injector is attached to a mixing device provided upstream of the SCR. The urea aqueous solution is injected from the injector to exhaust gas flowing through the mixing device to mix the urea aqueous solution with the exhaust gas within the mixing device. As a result, the urea aqueous solution is thermally decomposed by the heat of the exhaust gas to produce ammonia. The ammonia is used as a reductant in the SCR.
If the injected urea aqueous solution is not sufficiently mixed with the exhaust gas in the exhaust aftertreatment device, a part of the urea aqueous solution may be adhered on an inner wall of the mixing device of which outside is cooled by an external air, possibly causing shortage of ammonia in the SCR. Further, the urea aqueous solution turned to droplets on the inner wall of the mixing device may be crystallized to be deposited on the inner wall to hinder the flow of the exhaust gas. In order to solve the above problems, the mixing device sometimes has a double-tube structure including an outer tube and an inner tube. Since both inner and outer surfaces of the inner tube of the mixing device are in contact with the exhaust gas to be heated thereby, the urea aqueous solution adhered on the inner wall is thermally decomposed by injecting the urea aqueous solution to the inside of the inner tube. Thus, the urea aqueous solution can be restrained from being adhered on the inner wall in a form of droplets to be crystallized or deposited.
In addition, Patent Literatures 1 and 2 disclose a mixing pipe provided downstream an injector in a mixing device so that urea aqueous solution is sufficiently thermally decomposed. A plurality of openings are provided on an outer circumference of the mixing pipe. The exhaust gas flows into the mixing pipe through the openings to generate a turbulence or a swirl within the mixing pipe. The urea aqueous solution is injected from the injector into the flow of the exhaust gas to reduce the size of the urea aqueous solution particles, thereby facilitating the exhaust gas to be mixed with the exhaust gas and improving decomposition efficiency of the urea aqueous solution to ammonia.